Gaseous or vaporizable, i.e., short-chain, H-silanes SinH2n+2 are important starting materials for processes in which silicon is to be deposited on surfaces, for example, is CVD processes or for preparation of solutions (for ink-jet processes, for example). One decisive disadvantage of short-chain H-silanes is that the lower representatives (n=1,2) are gaseous and therefore only handleable in pressurized gas flasks. Further, all H-silanes up to a chain length of n=6 are without exception pyrophoric and therefore require appreciable safety precautions to be stored in a comparatively large amount. Yet the high vapor pressure of short-chain H-silanes is advantageous for gas phase processes since it ensures high concentrations of the silanes in the gas phase.
There is accordingly a need for a safe form of storing short-chain H-silanes and for a suitable method of releasing them as or when required.
It is known from DE 2139155, for example, that polysilanes SinH2n+2 having a chain length of n=7 or more are nonpyrophoric in air.
There are several known methods of releasing short-chain H-silanes from longer-chain polysilanes:
a) Thermal Decomposition
Polysilanes decompose into silicon and hydrogen at high temperature. However, that thermolysis starts at as low a temperature as close to 300° C. and then leads to hydrogen, short-chain silanes SinH2n+2 and also polymeric products (SiH<2)x, as known, for example, from R. Schwarz, F. Heinrich, Zeitschrift für anorganische und allgemeine Chemie 1935 (221) 277. The disadvantage with that method is the low yield of short-chain silanes obtained from the starting material since the thermolysis generates silicon-containing residues.
b) Catalytic Polymerization
It is know from R. C. Kennedy, L. P. Freeman, A. P. Fox, M. A. Ring, Journal of inorganic and nuclear chemistry 1966 (28) 1373, for example, that silanes having at least one Si—Si bond will in the presence of suitable catalysts such as lithium salts polymerize at comparatively low temperatures to form higher polysilanes (SiH2)x by elimination of SiH4. The disadvantage with that method is that only one molecule of SiH4 is formed per starting molecule SinH2n+2 during polymerization and the yield of short-chain material for safe storage substances with n>6 remains small. F. Feher, F. Ocklenburg, D. Skrodzki, Zeitschrift für Naturforschung 1980 (35b) 869 report that higher oligosilanes react with approximately equimolar amounts of AlCl3 on heating to eliminate SiH4 and a little di- and trisilane and form a yellow polymer having a composition of SiH0.98. A slowed polymerization is observed even at lower amounts of AlCl3 in aromatic solvents and at temperatures of not more than 85° C. Although the polymer contains less hydrogen, scarcely more than 50% of the starting silicon is released as short-chain silanes in the case of silanes where n>6.
c) Reaction of Polyfluorosilane with Hydrofluoric Acid
P. L. Timms, R. A. Kent, T. C. Ehlert, J. L. Margrave, Journal of the American Chemical Society 1965 (87) 2824 discloses a method that leads to formation of polysilanes from polyfluorosilanes (F2Si)x. The material is admixed with hydrofluoric acid and silanes having a chain length n up to 6 can be isolated in that only fully hydrogenated compounds SinH2n+2 are formed. The disadvantage is again the low yield of silanes since, during the reaction, H+ from the acid is formally reduced to hydride while at the same time SiO2 is formed, for example:4/x(F2Si)x+6H2O−>SiH4+3SiO2+8HF.
None of the cited processes demonstrate an efficient production of H-silanes in usable yields. The known methods also lack a storage material (storable form) that can be provided in the amounts needed.
It could therefore be helpful to provide a safe storable form for H-silanes and a method for recovering these.